1. Field of the Invention
The invention relates to a liquid crystal display with two spaced parallel support plates which enclose a liquid crystal layer that can be switched between two optically different states. The surfaces of the plates facing each other carry electrodes covered by a dielectric layer.
2. Description of the Prior Art
German Published Non-Prosecuted Application DE-OS No. 28 27 258 (U.S. Pat. No. 42 48 502) describes a liquid crystal display of this type.
The support plates of a liquid-crystal display are usually provided with a number of layers of optically different density, which reflect at their boundary surfaces part of the incident light. As a rule, these reflections are so strong that they reduce the picture contrast appreciably and make the not addressed segments stand out optically.
Work has therefore been done for years on an effective way to reduce reflections and a large number of proposals have been discussed, but a solution has not been arrived at that is acceptable from all angles. The reason for this is primarily that the display area was to be kept free of reflections in the regions that are without electrodes as well as over the electrodes, and that this demanding condition must be met in a simple manner, since cost considerations have come to play a central role.
The cited Offenlegungsschrift No. 28 27 258 describes, among other things, a type of display in which the electrodes are enclosed between a continuous double layer, consisting of a protective layer which is in direct contact with the support plate, and an orientation layer facing the liquid crystal. The conductive layer and the orientation layer must then have a thickness which depends in a characteristic manner on the wavelength of the light as well as on the index of refraction of the substrate, the electrode, the orientation layer and the liquid crystal, while the protective layer should have, as far as possible, the same optical density as the support plate. With such a coating, the reflections can be suppressed sufficiently on the entire display area without special extra effort because, first, a special layer for orienting the liquid-crystal molecules and for shielding the electrodes against the alkali ions of the substrate is necessary anyhow, and second, the thicknesses of these layers may deviate distinctly from the ideal values. On the other hand, the antireflection effect is not always sufficient if the display works with unpolarized light. This relates to the fact that the birefringent liquid crystal has, for radiation with different directions of polarization, different optical densities, but the layer thicknesses are optimized under the assumption that the liquid-crystal layer has only a single index of refraction.
Interference in the display could, of course, also be suppressed by an antireflection coating inserted between the support plate and the electrodes. In this manner, however, good values of attenuation with unpolarized radiation are obtained only if the coating is built-up from a multiplicity of carefully matched individual layers. If, in order to limit the manufacturing effort, only two individual layers are used, the not addressed segments often have a tinge of color (U.S. Pat. No. 3,736,047).
Therefore, an efficient antireflection technique is lacking, by which a liquid-crystal display can be interference-suppressed optically perfectly on the entire area if the display is also operated with unpolarized light. To close this gap which is troublesome particularly especially because it has been possible in the meantime to develop competitive liquid-crystal displays without polarizers, a display with the features according to the invention is proposed.